In general, a portable electronic device is equipped with a DC-DC converter used to convert the voltage generated by the battery to the desired power supply voltage of the electronic circuit. The battery voltage varies as a function of the amount of remaining power. For example, the voltage of a lithium ion battery varies in the range of 3-4.2 V. Also, if an external power supply, such as an AC adapter, is used without an incorporated battery, a voltage of about 5 V may be input instead of the battery voltage. The DC-DC converter can generate a constant output voltage despite the variation in the input voltage.
There are three types of DC-DC converters: step-up, step-down, and step-up/step-down. The step-down type is used when the lower limit of the input voltage is higher than the target value of the output voltage. The step-up type is used when the upper limit of the input voltage is lower than the target value of the output voltage. The step-up/step-down type is used when the input voltage may be lower or higher than the output voltage.
In order to reduce consumption of the battery in portable electronic devices as much as possible, a standby mode is adopted, in which part of the circuit operation is stopped when the device is not in use. Since the load on the DC-DC converter is very low in the standby mode, a pulse frequency modulation (PFM) system can save more power than a pulse width modulation (PWM) system that constantly switches at a prescribed period.
An example of the PFM type DC-DC converter (also known as a switching regulator) is disclosed in JP Patent Publication 11-235023 (1999) (JP Patent Application 10-036181 (1998)). This reference discloses a technology for reducing the ripple voltage when the power supply voltage is high by adopting power supply voltage dependence for the PFM duty ratio. An example of the PWM type switching regulator is disclosed in JP Patent No. 3556652. This reference discloses a PWM type switching regulator with high voltage conversion efficiency.
In general, in a DC-DC converter, a voltage corresponding to the difference between the input voltage and output voltage is applied to an inductor that manages the energy converting operation. In the PFM system, the period in which the voltage is applied to the inductor is fixed at one cycle of the switching operation. The output voltage is controlled by varying the repetition rate of the fixed period. Since the PFM type switching operation is basically an intermittent operation, it is difficult to keep the output voltage constant for high loads when the power consumption of the device connected to the output terminal is high. However, the output voltage can be kept constant at low loads, and the power consumption is low and the efficiency is high because of the intermittent operation. On the other hand, in the PWM system, the period of the switching operation is fixed, and the output voltage is controlled by controlling the duty ratio in which the switch-on pulse width varies during the period. Since the PWM type switching operation is basically an intermittent operation, it can handle high loads. In this case, the high power consumption will not become a serious problem since the power consumption of the load is also high. For low loads, however, the high power consumption will lead to low efficiency.
A switching regulator that switches the PFM system and PWM system corresponding to the magnitude of the load is known. As described in Japanese Patent No. 3556652, the PWM type switching power supply device has a GM amplifier, which feeds back its output voltage and controls the duty ratio of the switching operation corresponding to the level of the output voltage. A capacitor with a relatively large capacitance (for example, on the order of nF) used for phase compensation is connected to the output of the GM amplifier. Since the charging voltage of this capacitor is held when the PWM operation is stopped immediately before switching to PFM operation, the charging voltage does not have the appropriate value when the system enters PFM operation or when it switches from the PFM operation to PWM operation. Consequently, it takes time to start and stabilize the PWM feedback circuit (analog circuit). If the operating mode is changed from PFM to PWM before the analog circuit stabilizes, the output voltage will be overshot or undershot, making it impossible to output a continuous constant voltage. Therefore, a time delay is needed until the system switches from PFM operation to PWM operation after a high load is detected. In this case, however, the operating frequency of the PFM control circuit increases instantaneously corresponding to the high load, making it difficult to make a smooth mode transition.
An object of the invention is to solve the aforementioned problem by providing a switching power supply device that can make a smooth transition from the intermittent control mode to the continuous control mode, has a stable output voltage, and can suppress unnecessary power consumption as much as possible.